Significantly improved peak rates of 1 Gbps in the downlink and 500 Mbps in the uplink are required for a Long Term Evolution Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. Carrier Aggregation (CA) is introduced to the LTE-A system to accommodate the required improved peak rates, compatibility with the LTE system and full use of spectrum resources.
Carrier aggregation refers to presence of a plurality of component carriers in both the uplink and the downlink in a cell instead of a scheme in which there is only one set of carriers in the LTE system and earlier radio communication systems. In the system with carrier aggregation, the component carriers may be consecutive or inconsecutive, the maximum bandwidth of each component carrier is 20 MHz for compatibility with the LTE system, and the bandwidths of the component carriers may be the same or different.
A random access of a user equipment in the LTE system arises generally in the following several scenarios.
In a first scenario, the user equipment in a Radio Resource Control Idle (RRC_IDLE) status accesses the system, which is also referred to as an initial access.
In a second scenario, the user equipment initiates a random access after a radio link fails, which is also deemed as an initial access.
In a third scenario, a random access is required for the user equipment during a cell handover.
In a fourth scenario, the user equipment in a Radio Resource Control Connected (RRC_CONNECTED) status has downlink data incoming
In a fifth scenario, the user equipment in an RRC_CONNECTED status has uplink data incoming.
In the third and fourth scenarios, the user equipment can have a random access in a non-contention random access mechanism, and FIG. 1 illustrates a non-contention random access procedure which generally involves the following three messages.
For a message 0: an eNodeB (eNB) assigns a Random Access Preamble (ra-PreambleIndex) for a non-contention random access and a Physical Random Access Channel Mask Index (ra-PRACH-MaskIndex) for the random access to the user equipment. For a non-contention random access due to incoming downlink data, the ra-PreambleIndex and the ra-PRACH-MaskIndex are transmitted to the user equipment over a Physical Downlink Control Channel (PDCCH), and for a non-contention random access due to a handover, the ra-PreambleIndex and the ra-PRACH-MaskIndex are carried in a handover command transmitted to the user equipment.
For a message 1: the user equipment transmits the random access preamble specified in the message 0 to the eNodeB over a PRACH resource specified by the ra-PRACH-MaskIndex according to the received ra-PreambleIndex and ra-PRACH-MaskIndex.
For a message 2: the eNodeB transmits a random access response to the user equipment.
A flow of a cell handover between different eNodeBs performed in the same Mobility Management Entity (MME) is briefly explained below as illustrated in FIG. 2.
Firstly a user equipment reports a measurement, a source eNodeB decides on a handover and transmits a handover request command to a target eNodeB after deciding the handover as the result of the handover decision, the target eNodeB decides on admission upon reception of the handover request command transmitted from the source eNodeB and transmits a handover request acknowledge message to the source eNodeB if admission is allowed, and the source eNodeB transmits a handover command to the user equipment upon reception of the handover request acknowledge message.
The Handover Request Acknowledge message in the step 6 of FIG. 2 includes a Target eNB to Source eNB Transparent Container which is a transparent container to be transmitted to the user equipment as a part of the handover command and which includes an ra-PreambleIndex and an ra-PRACH-MaskIndex dedicated to a non-contention random access if the non-contention random access is selected.
The ra-PRACH-Mask Index identifies a PRACH resource available to the random access of the user equipment, which is a two-dimension resource with the dimensions of Sub-frame/PRACH Resource. In the LTE system, a corresponding relationship between an ra-PRACH-Mask Index and a PRACH resource at the physical layer is as depicted in the table below in which an PRACH Resource Index represents the index of a PRACH resource in a radio frame, and the index of a PRACH resource corresponds to a specific physical resource configured at the physical layer.
PRACHAvailable PRACH resourceAvailable PRACH resourceMask(in a Frequency Division(in a Time Division DuplexIndexDuplex (FDD) system)(TDD) system)0AllAll1PRACH Resource Index 0PRACH Resource Index 02PRACH Resource Index 1PRACH Resource Index 13PRACH Resource Index 2PRACH Resource Index 24PRACH Resource Index 3PRACH Resource Index 35PRACH Resource Index 4PRACH Resource Index 46PRACH Resource Index 5PRACH Resource Index 57PRACH Resource Index 6Reserved8PRACH Resource Index 7Reserved9PRACH Resource Index 8Reserved10PRACH Resource Index 9Reserved11Even-numbered sub-frame;Even-numbered sub-frame;The first PRACH resourceThe first PRACH resourceof each even sub-frame;of each even sub-frame12Odd-numbered sub-frame;Odd-numbered sub-frame;The first PRACH resourceThe first PRACH resourceof each odd sub-frame;of each odd sub-frame;13ReservedThe first PRACH resourceof each sub-frame;14ReservedThe second PRACH resourceof each sub-frame;15ReservedThe third PRACH resourceof each sub-frame;
With the introduction of CA, a plurality of component carriers are supported concurrently in the uplink, so the user equipment can not determine a specific uplink component carrier over which the random access preamble is transmitted to the eNodeB. If the user equipment transmits the random access preamble over an uplink carrier different from an uplink carrier over which the eNodeB receives the random access preamble, the random access procedure may not be performed; or if the user equipment transmits the random access preamble concurrently over the plurality of uplink component carriers, an uplink resource may be wasted although the eNodeB can receive the preamble.